Remediation of contaminated particulate materials

ABSTRACT

A process for the remediation of contaminated particulate materials by the addition of an environmentally benign, carbonaceous fuel source in low concentration to enable or enhance smoldering combustion. The process may be applied to both in situ and ex situ treatments. In an ex situ smoldering process for the remediation of contaminated particulate materials in a continuous manner, contaminated feed is introduced near the top of a treatment unit and treated product emerges near the bottom. A smoldering front is maintained in the unit, fed by the fuel in the contaminated particulate material and a supply of combustion-supporting gas, such as air.

FIELD OF THE INVENTION

The presently disclosed subject matter relates to the remediation ofparticulate materials contaminated with organic compounds. Inparticular, the presently disclosed subject matter relates to a processfor the application of smoldering combustion to remediate contaminatedparticulate materials.

BACKGROUND OF THE INVENTION

Contamination of particulate materials (e.g., soils, sediments, sludges,or muds) can arise from hazardous materials being spilled, leaked,discharged, co-processed, or buried at a site, or from intrusion ofcontaminants from offsite sources. As an example, leaking undergroundstorage tanks have contaminated the soil at sites with petroleumhydrocarbons and lead, and caused similar contamination of adjacentsites through migration in the subsurface. In general, contaminantswhich may be found in particulate materials include liquids, which mayhave associated vapors, and solids. Contaminants may be physically orchemically attached to particles, or may be present as a separate phasebetween particles, such as non-aqueous phase liquids (NAPLs). Liquidcontaminants include petroleum hydrocarbons, coal tars, and industrialsolvents, while solid contaminants include salts, metals, and organicmaterials (e.g., explosives, pesticides).

Remediation of contaminated particulate materials implies the removal ofcontaminants and their impacts for the general protection of health andfor the benefit of the environment. For example, remediation at formerindustrial (“brownfield”) sites may be a prerequisite for redevelopmentfor residential, commercial, or new industrial use. Remediation isgenerally subject to an array of regulatory requirements, and also canbe based on assessments of human health and ecological risks in light ofplanned future use.

Site remediation methods addressing contamination can be classified ingeneral terms as: (1) ex situ methods in which the particulate materialis displaced and treated or disposed of in a waste facility; and (2) insitu methods in which the particulate material remains in place fortreatment. Stabilization, solidification, or containment, while notremediation methods in themselves, may also be used to prevent thecontamination from becoming more widespread. Treatment methods fororganic contaminants are numerous and varied, with examples including:thermal approaches like incineration and thermal desorption;washing/flushing with aqueous solutions or organic solvents;bioremediation, in which microbial activity is stimulated to achieveenhanced biodegradation; and chemical oxidation, typically with anaqueous solution or gas. Combinations of methods are also common, suchas washing with thermal treatment of separated, highly contaminated fineparticles.

Of the thermal remediation methods for particulate materials,incineration is generally the most effective for destroying organiccontaminants due to the high temperatures achieved. However, hightemperatures also have greater associated fuel costs and tend to degradenative properties of soils and sediments. Due to the range of wastes fedinto incinerators, their emissions must also be carefully monitored andcontrolled (e.g., through temperature and filtration). Thermaldesorption, in which organic contaminants are desorbed from particulatematerials and combusted in a burner, is a leading alternative toincineration. Thermal desorption is a cooler, less harsh treatment thanincineration, but may be ineffective for particulate materials with highmoisture contents or contaminant levels. While not as fuel-intensive asincineration, thermal desorption also has substantial fuel costsassociated with both the desorbing and combusting of contaminants.

Remediation technologies are frequently benchmarked by cost to offsitedisposal at a waste facility, which relocates contaminated particulatematerials to an engineered site for long-term storage. Offsite disposalis often the most economical option for these materials and has arelatively low risk of failure in reaching regulatory criteria at asite. Except when transportation is impractical due to distance orquantity of material, few technologies can routinely compete with itscombination of reliability and cost effectiveness. One of the mostcommonly employed alternatives to offsite disposal for organiccontamination is bioremediation, which can cost roughly half the price.While bioremediation provides significant savings over offsite disposal,its application is generally restricted to particulate materials withrelatively low levels of contamination, especially in the morerefractory hydrocarbons. There is therefore a need for a cost-effectiveremediation technology that is effective when bioremediation cannot beapplied.

Smoldering, which is a flameless combustion process, is a promising newapproach for remediating particulate materials containing organiccontamination. Smoldering may be sustained in a particulate materialprovided sufficient fuel is present. This process occurs naturally, forexample, in underground peat fires. However, organic contaminants canalso provide sufficient energy for self-sustaining smoldering combustionunder the right conditions. Generally, these conditions include highenough contaminant concentrations, a supply of air, adequate retentionof heat, and an initial source of heat to ignite the smoldering front.If these conditions are met, smoldering can be used as a process toremediate particulate materials, virtually eliminating all organiccontaminants.

Smoldering combustion can be initiated in an in situ approach byactively heating a small region of contaminated particulate materialbelow the surface and introducing air once that region has reachedignition temperature (typically 200-400° C.). The heater may then bedeactivated, while the air supply is maintained to sustain a smolderingfront, which propagates through the bed of particulate materialdestroying contaminants. Provided there is sufficient fuel for theprocess in the particulate material, smoldering can be self-sustainingin the sense that no further active heating is required after ignition,as the contaminants themselves supply the heat required for theirongoing destruction.

Smoldering has the potential to provide thorough contaminant removal ina cost-effective process. Unlike bioremediation, smoldering is capableof remediating particulate materials with high levels of organiccontaminants, including the more refractory contaminants associated withevents like crude oil spills. It is, moreover, facilitated by highercontaminant concentrations and is therefore naturally suited to heavilycontaminated particulate materials. In addition, the cost of smolderingon a proportionate weight basis may be similar to bioremediation as aresult of the savings on fuel costs, which are significant costs forother thermal remediation technologies (e.g., incineration and thermaldesorption). However, like other forms of thermal remediation,smoldering thoroughly removes combustible contaminants, enablingstringent remediation standards to be met. Smoldering therefore offersthe possibility of a thorough, robust, and cost-effective technology forremediating particulate materials contaminated with organic compounds.

Smoldering has been previously considered as a beneficial adjunct tothermal desorption. For example, it was recognized that hot gases usedto desorb hydrocarbons could also combust molecules in the soil, asdescribed in U.S. Pat. No. 5,193,934. In addition, the use of porousburners has been described in WO 95/3045 and WO 95/34349 to destroydesorbed hydrocarbons in a smoldering process with the heat ofcombustion recycled to assist in subsequent desorption. The use ofsmoldering as a primary strategy for soil remediation has been describedby Gerhard et al. in Proc. Combust. Inst. 32, 1957 (2009), Environ Sci.Technol. 43, 5871 (2009), and Environ Sci. Technol. 45, 2980 (2011), aswell as in situ methods described in CA 2 632 710 and US 2009/0180836,which have been the focus of reported field work to date. In certainrespects, namely the use of subterranean combustion, in situ smolderingresembles the enhanced oil recovery method known as in situ combustionor fireflooding, described, for example, by Moore et al. in Fuel 74,1169 (1995).

More recently, US Patent Application Publication No. 2012/0272878 toGrant et al. describes the application of smoldering for the volumetricreduction of organic liquids. US Patent Application Publication No.2012/0288332 to Thomas et al. describes a method for remediating porousmaterials by fuel-assisted smoldering. While both disclose methods forenhancing smoldering with supplemental fuel sources, the focus is onfuel sources such as oily waste and petroleum hydrocarbons that areunlikely to confer environmental benefits to the smoldered products(e.g., treated soils and sludges), and which may, in some instances, bedeleterious if complete destruction of the supplemental fuel source isnot achieved or if more hazardous emissions are produced.

SUMMARY OF THE INVENTION

In order for smoldering to be self-sustaining, and therefore morecost-effective, adequate fuel must be present in the contaminatedparticulate material. This fuel can be the target organic contaminantsor natural organic matter such as lignite or peat. Particulate materialswith insufficient levels of these fuels may nonetheless have high enoughorganic contaminant concentrations to be considered contaminatedaccording to regulatory standards. If the particulate material does notcontain sufficient fuel to permit self-sustaining smoldering combustion,an additive in the form of an environmentally acceptable fuel source canbe introduced to increase its fuel content and promote smoldering. Thismay be done regardless of whether smoldering is carried out in situwithout displacement of the contaminated particulate material or ex situin engineered mounds or pits or a reactor. In addition, the use of suchadditives may be beneficial for the remediation of any particulatematerial through smoldering by increasing treatment temperatures formore complete contaminant oxidation; providing a baseline amount of fuelto mitigate fluctuations due to heterogeneity in the particulatematerial; or improving the quality and beneficial reuse of smolderedproduct. The additives described herein are environmentally benignsubstances rich in organic carbon; they are added in quantitiessufficient to enable effective self-sustaining smoldering. Examples ofsuitable additives include peat and humalite.

According to the present invention, the additive for promotingsmoldering is selected to have a total organic carbon concentration(weight of non-carbonate carbon in a sample divided by dry sampleweight) typically of at least 25 weight percent, preferably above 35weight percent and, in order to enable effective self-sustainingsmoldering, it is added to the particulate material in an averageconcentration in the particulate material, following introduction, of upto 10 weight percent, typically 1 to 5 weight percent, depending on theamount of available smoldering supporting material present in theparticulate material prior to the addition. The additive may bedistributed heterogeneously within a treatment bed for an in situapplication, in which case the average concentration is based on theentire bed to be treated.

When the addition of the additive is used for in situ remediation, itwill normally be a liquid such as biodiesel or bioethanol in order tofacilitate injection through one or more injection wells located in thecontaminated region. Smoldering may then be initiated at a locationproximate the location at which the liquid fuel is injected and a supplyof combustion-supporting gas, normally air, is maintained to sustain thesmoldering front as it progresses through the treatment bed. The frontmay assist in distributing the liquid additive through the bed, in amanner akin to the transport of oils seen in fireflooding. Solidsmoldering-promoting additives may be added to the treatment bed byinsertion through boreholes, trenches, or excavations, after whichsmoldering may be ignited and sustained by the injection of air throughinjection wells.

In addition, we have developed a process as well as a process unit forex situ smoldering which can be used for the remediation of contaminatedparticulate materials either with or without a smoldering-promotingadditive, depending on the smoldering characteristics of the soil. Theex situ remediation process conducted in this unit is distinguished bybeing of a continuous mass-flow design. While the ex situ approach hasthe added expenses associated with bulk displacement (e.g., soilexcavation or sediment dredging) the advantages in process control aresignificant. They include preprocessing of particulate materials (e.g.,shredding, intermixing, drying, dewatering, or adding fuel/diluent),enhanced air distribution for more uniform treatment throughout the bed,confinement of smoldering to the vessel (i.e., lower risk of spreadingto the surroundings—a particular hazard in forested areas or withsurrounding peat bogs), and easier handling of emissions. In addition,the continuous mass-flow design improves efficiency over a batch systemby eliminating downtime (increasing throughput) while maintaining astable smoldering process.

According to the present invention, the ex situ process for theremediation of contaminated particulate materials comprises: (i)removing the contaminated particulate material from its location in theground, (ii) transferring the contaminated particulate material removedfrom the ground and admitting it into a remediation unit comprising avertically extensive smoldering combustion vessel having an inlet at thetop for contaminated feed and an outlet at the bottom for remediatedproduct to be withdrawn from the vessel, (iii) heating a region ofloaded feed in the vessel, (iv) admitting combustion-supporting gas intothe loaded feed to ignite a smoldering combustion front and enable it toprogress through the loaded feed in the vessel, and (v) withdrawingremediated, smoldered product from the bottom of the vessel andadmitting additional contaminated feed at the top of the vessel tomaintain the smoldering front at a vertical location in the vesselbetween the top and the bottom of the vessel.

The process unit for the remediation of particulate materials comprises,briefly, a vertically extensive smoldering combustion vessel having aninlet at the top for contaminated feed to be remediated by smoldering,an outlet at the bottom for treated, smoldered product to be withdrawnfrom the vessel, means for heating a region of loaded feed within thevessel, and means for injecting combustion-supporting gas into theloaded feed to ignite and sustain smoldering combustion. The vessel willgenerally have an upper section in which smoldering takes place and alower section which channels the smoldered product towards the outlet.In one embodiment, the lower section will be a mass-flow funnel, as isknown in the field of bulk solids handling, emptied by an auger or screwconveyor at its bottom, which transfers smoldered product to the outlet.This product withdrawal mechanism is capable of providing mass flow, inwhich the treatment bed in the upper section is emptied uniformly, asopposed to funnel flow, where emptying of the upper section occursdisproportionately in the center. By achieving mass flow during productwithdrawal, the risk of disturbing or even extinguishing the activesmoldering front is minimized. Further preferred features of the unitare described in more detail below.

DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic illustration of a continuous smoldering system forex situ remediation of contaminated particulate materials.

FIG. 2 is a graphical representation of the temperature profiles fromsmoldering tests on soils prepared with different concentrations ofbiodiesel.

FIG. 3 is a graphical representation of the residual heavy hydrocarbon(C16-C34) concentrations in soils smoldered with differentconcentrations of biodiesel.

FIG. 4 is a graphical representation of the temperature profilesobserved during benchtop smoldering tests on soils prepared withdifferent concentrations of humalite.

FIG. 5 is a graphical representation of the hydrocarbon contaminantconcentrations in a mixture of two soils before and after smoldering.

DETAILED DESCRIPTION

Smoldering Unit

The functioning of the smoldering-promoting additive may be betterunderstood from a description of the ex situ smoldering process unit andthe process which is carried out in it. A schematic diagram of thecontinuous smoldering system for ex situ remediation of contaminatedparticulate materials is shown in FIG. 1. During operation of thesystem, a conveyor 1, which could be based on a belt or auger, deliverspretreated feed to a hopper 2. The pretreatment may consist ofshredding, drying (possibly with heat recycled from the smolderingunit), dewatering (e.g., treating dredged sediments with aplate-and-frame filter press), removing large rocks or debris,homogenizing, mixing different feeds together, or mixing in a substanceto enhance smoldering performance, e.g., a smoldering-promoting additiveor a less combustible diluent. The level in the hopper is monitored by asensor 3, which actuates the conveyor system when the feed in the hopperis below a preset level. Feed exiting the hopper passes through ashredder 4 to improve packing and air distribution before entering theopen top of the smoldering vessel 5, which is configured forbottom-to-top front propagation. Inside the chamber, loaded feed isevenly distributed by a rake or plow system 6, and the loaded feed levelis controlled using another sensor 7, which actuates the shredder.

The smoldering chamber has ports on the sides for air spargers 8,heating cables 9, and thermocouples 10. The air spargers are perforatedpipes (perforated on the top side) located in a plane, typically nearthe bottom of the chamber. To prevent clogging, the perforations arecapped with conical deflectors (not shown). The heating cables lie in aplane somewhat above the air spargers, and may be retractable since theyare only required for ignition. The thermocouples are verticallydistributed throughout the treatment bed to monitor temperatures duringboth the initial heating stage and continuous smoldering process so asto monitor the position and progress of the smoldering front; thethermocouples may also be distributed radially to determine thetemperatures across the bed. If bulk solid flow through the systemdisturbs the thermocouples, an alternative arrangement would be in abundle along the vertical axis of the reactor, passing through the shaftof the rake or plow system.

The smoldering chamber 5 has two more external connections. At the topof the chamber, a pipe 11 is attached for collecting the off-gas fortreatment to remove noxious gases and any solid particulates to complywith air emission standards. The pipe is desirably under negativepressure to draw the gas in. The gases may subsequently pass through awater knockout tank and volatile organic compound (VOC) removal system(e.g., a carbon adsorption bed or burner). The second connection islocated at the bottom, where the chamber tapers as a mass-flow funnel12, which terminates over a channel or pipe housing an auger or screwconveyor 13 for removing smoldered product. The combination of amass-flow funnel with an auger or screw conveyor is an establishedsystem in the bulk solids handling industry for achieving evenwithdrawal (mass flow) over the cross section of a vessel, which willcause less disruption to the active smoldering front. The ability tovary the rotational speed of the auger provides a high degree of controlover the rate of product withdrawal. The auger is actuated based oninput from the thermocouples, as described below. Alternativeconfigurations for achieving mass flow from the chamber include havingmultiple augers in the channel at the base of the funnel or eliminatingthe funnel and having what is known in the bulk solids handling industryas a “live bottom” at the base of the chamber, in which an array ofaugers in a horizontal plane is used to achieve mass flow, maintaining astable and even smoldering front in the upper region of the vessel.

Operation of Smoldering Unit

During operation, the smoldering chamber 5 is first filled with acontaminated particulate material to be treated, the particulatematerial optionally mixed with one or more of the following: asmoldering-promoting additive; a complementary particulate material thatmodifies aggregate fuel concentration, thereby altering the temperatureor rate of smoldering, or aggregate texture for improvedcombustion-supporting gas flow or distribution. Examples ofcomplementary particulate materials include other contaminatedparticulate materials in need of treatment and less combustiblediluents, such as sand or previously smoldered product. Admixing withthe primary contaminated particulate material may be accomplished bymeans of an additional feeder, which may be integrated with the conveyoror a separate component of the feed system. The additional feeder maybe, for example, a drip line, injector, or sprayer for liquid additivesor a screw feeder, shaker (above the conveyor) or separate conveyor forsolids, such as solid additives and complementary particulate materials.The heaters are then activated to raise the temperature of nearby loadedfeed to 200-400° C. for ignition. Air is subsequently introduced throughthe spargers, possibly blended with nitrogen to moderate the onset ofsmoldering combustion. Once the smoldering process is established (i.e.,rapid temperature rise near the heaters is detected), the heaters may bedeactivated and retracted from the chamber to facilitate bulk solidflow. At this point, only a supply of air is needed to sustain thesmoldering front, which propagates upward through the treatment bed.

Without further intervention, the smoldering front would terminate whenit reached the top of the treatment bed or when the air supply wasturned off. Only the loaded feed within the chamber would then beremediated and the process would have to be repeated for successivebatches. To eliminate the downtime associated with successive batches,the system runs in one of two continuous modes. In fully continuousmode, the speed of the smoldering front is matched by the rates of thefeed supply system (feed conveyor 1 and shredder 4) and the withdrawalauger 13 so that the front remains in the same substantially fixedvertical position in the vessel. The speed of the front is determinedfrom temperature data at different heights, and feed is continuouslyprovided from the top of the chamber while being continuously withdrawnfrom the bottom. In semi-continuous or intermittent mode, the augersystem keeps the smoldering front in a certain vertical region or “hotzone” 14 marked by a lower vertical location and an upper verticallocation. When the front reaches the upper thermocouple of this region,the auger is activated until enough product has been withdrawn to alignthe front with the lower thermocouple. This process is then repeated asnecessary while the overall quantity in the chamber is kept at thepre-set level by the feed supply system. Depending on the speed ofpropagation of the smoldering front, either one of these continuousmodes could be more convenient.

As demonstrated in the Examples, relatively high temperatures may beachieved in the process, typically in excess of 600° C. at the advancingsmoldering front; these temperatures will destroy most organiccontaminants, though inorganic contaminants (e.g., metals), if present,would typically not be removed by this process (exceptions may includerelatively low-boiling metals, such as mercury). However, for manycontaminated particulate materials, removal of organic contaminants issufficient to achieve remediation.

Smoldering-Promoting Additives

Environmentally benign additives which promote smoldering combustion maybe added to contaminated particulate materials to facilitate theremediation process. A smoldering-promoting additive can be effective toenable a particulate material that is unable or only intermittently ableto sustain smoldering combustion to consistently sustain smolderingcombustion; to enable smoldering combustion to take place at a highertemperature, more favorable to contaminant destruction; or to improvethe quality and beneficial reuse of smoldered product. The additive maybe used in ex situ processes, for instance, as described above, or incombination with an in situ process in which the additive may beinjected or otherwise incorporated within the contaminated particulatematerial in place.

Both liquid and solid smoldering-promoting additives may be used; liquidadditives are particularly well suited for in situ application since theliquid may be injected into the ground through injection wells atsuitably spaced intervals. Smoldering-promoting additives have thecapability both to extend the application of smoldering technology andto improve process performance without the risks associated with mixingin contaminants (e.g., oily waste or petroleum hydrocarbons) as fuelsources, which include failure to achieve complete removal and morehazardous emissions. The benefits of smoldering-promoting additives willderive from: permitting contaminated particulate materials to be treatedusing smoldering combustion that would otherwise have insufficient fuelfor a self-sustaining process; enabling higher smoldering temperaturesto be achieved, which provide greater contaminant oxidation and simpleremissions handling as a result of lower levels of unoxidized orpartially oxidized contaminants in the off-gas; and compensating forheterogeneity in the distribution of fuel in contaminated particulatematerials by providing a base amount of fuel to sustain a steady,uninterrupted smoldering process. In addition, the quality andbeneficial reuse of the smoldered product may be enhanced in at leastone of the following respects: improved texture, e.g., permeability;improved ability to absorb or retain water and other nutrients;increased residual natural organic matter.

The additives are typically used at a concentration of less than 10weight percent of the particulate material but higher amounts may beused if necessary, although the economics of the process may beadversely affected. Lower amounts may be appropriate, e.g., 1 to 5weight percent, when the contaminants or natural organic matter presentin the particulate material provide substantial fuel for the process. Ingeneral, the smoldering-promoting additives may be characterized asenvironmentally benign, carbon-rich substances with total organic carbonconcentrations of at least 25 weight percent, typically of at least 35weight percent. Solid additives could be, for example, peat, humalite orbiochar. The solid smoldering-promoting additives may be ground orotherwise broken down prior to admixing with the contaminatedparticulate material in order to achieve a reasonably even mixture withuniform smoldering properties.

As noted above, the carbonaceous additive which is distributed throughthe particulate material may be a liquid (at the prevailing atmospherictemperature and pressure) or a solid material. Suitable carbonaceousliquids, typically with a total organic carbon concentration of 35 to 85weight percent, may include alcohols, e.g., bioethanol, biobutanol orhigher alcohols if sufficiently economically attractive, esters such asfatty-acid methyl esters, conveniently available in large quantity asbiodiesel, or other materials which are combustible under appropriateconditions and biodegradable in the environment, e.g., fatty acids orother fatty-acid derivatives. Fatty acids and fatty-acid derivatives mayalso be solid or semi-solid, e.g., waxy materials, depending on theprevailing conditions. Liquid materials may be introduced into atreatment bed as such or in the form of solutions or suspensions withthe additive contained in a carrier, such as water. Depending on themobility of the liquid into the bed, the liquid may be introducedthrough percolation over a period of time before smoldering isinitiated. Alternatively, distribution may be achieved through one ormore injectors extending into the ground. In the case of low-mobilityadditives, such as fatty-acid derivatives, high additive concentrationsproximate to injection locations may be distributed through thetreatment bed under the action of the smoldering front. This migrationof the additive would occur ahead of an approaching smoldering front, astemperatures rise and water is converted to steam. The process iscomparable to the one in which oils migrate during fireflooding.

A preferred class of smoldering-promoting additives includes peat,leonardite (an oxidized derivative of lignite), and humalite (aderivative of sub-bituminous coal). These materials are rich in humicsubstances (a mixture of humic acids, fulvic acids, and humin formedfrom decayed organic matter), typically have total organic carbonconcentrations in the range of 30 to 50 weight percent, and are commonlyused as soil conditioners, a practice which demonstrates environmentalcompatibility with soil. Additionally, residual amounts of theseadditives in treated soil or other particulate materials may improveproperties such as organic content, nutrient retention, texture, abilityto chelate metals, and microbial activity. From this class of additives,humalite is particularly attractive because it tends to be denser thanpeat and possess lower sulfur, metals, and ash content than leonardite.Beyond this class, other suitable solid smoldering-promoting additivesinclude lignite, biochar, biosolids, compost, corn stalks, chaff,chopped straw, rice hulls, shredded wood or bark, wood chips, choppedbagasse, and other agricultural and forestry wastes, depending on theirlocal availability and economics.

For ex situ smoldering applications, solid or liquid additives canreadily be mixed or dispersed into particulate materials as apre-treatment step or as part of other pre-treatment steps, such as soilshredding. Additives may also be mixed or dispersed into particulatematerials during transfer to a smoldering treatment chamber, forexample, by a separate feeder for solids or a spray nozzle for liquids.For in situ smoldering, liquid additives may be injected or otherwisedispersed into the ground and solid additives may be incorporated intothe ground through boreholes, trenches, or excavations.

The following scenarios illustrate some applications of the remediationprocedure using solid and liquid smoldering-promoting additives.

Scenario 1: Large quantities of soil excavated from the site of a formerpetroleum refinery have been subject to bioremediation. A significantfraction of the bioremediated soil still exceeds regulatory limits forheavy hydrocarbons (hexadecane and heavier). Transfer of this soil to awaste facility would be costly and possibly impractical depending on thequantity involved. A continuous ex situ smoldering unit can be broughton-site to treat the soil at a cost well below offsite disposal at awaste facility, however, the fuel content of the soil is variable andsometimes below the self-sustaining threshold for smoldering. As thesoil is fed into the smoldering reactor of the unit, the feed stream ismixed with fine biochar to achieve a concentration of 2 dry weightpercent biochar. The entire mass of contaminated soil can now be treatedin a reliable and economical process, with residual biochar functioningas a soil amendment to improve treated soil quality.

Scenario 2: The soil at a former production site in the far North has abroad spectrum of hydrocarbon contamination due to past exposure tocrude oil. The remote location and cold climate at the site eliminateoffsite disposal at a waste facility and bioremediation, respectively,as economic site remediation options. The soil is found to havesufficient fuel to sustain smoldering combustion, but its clayey texturenecessitates shredding to improve air distribution before smoldering canbe applied. A batch ex situ smoldering unit at the site effectivelydecontaminates the soil, but efficiency is lagging and costs areelevated due to frequent saturation and changing of the carbonadsorption bed used to filter harmful emissions. To address theseissues, peat is added to the soil fed through the shredder to achieve apost-shredding concentration of 4 weight percent peat. Smolderingtemperatures are consequently increased by 200° C., significantlyreducing incomplete combustion of contaminants and saturation of thecarbon adsorption bed.

Scenario 3: An in situ project to remediate soil with fuel-oilcontamination uses soil flushing with a surfactant at an array ofinjection points. Soil core samples taken after treatment show thatresidual heavy hydrocarbons are moderately above regulatory standards atmany locations and distributed in a discontinuous pattern. To allowtreatment of the heavy hydrocarbons without site excavation, biodieselis injected below ground using the system in place for soil flushing.The biodiesel enables in situ smoldering to be applied to remediate thesite by elevating fuel concentrations to self-sustaining levels andproviding a path for the smoldering front between disconnectedcontaminant volumes. Non-combusted biodiesel remaining in the groundafter treatment rapidly biodegrades over the next two months.

Scenario 4: Mechanically-dredged hydrocarbon-contaminated sediment froman industrial waterway has a moisture content of 60 weight percent. Thesediment is dewatered using a plate-and-frame filter press to 35 weightpercent water. Due to high transport costs and tipping fees, disposal ofthe dewatered sediment at a waste facility is cost prohibitive, so acontinuous ex situ smoldering system is set up on-site to treat thehydrocarbon contaminants. Due to the high moisture content, thedewatered sediment has insufficient fuel to enable self-sustainingsmoldering combustion. The deficit in fuel is balanced by the additionof humalite to the sediment during feeding of the sediment into thesmoldering chamber. As a result, on-site treatment and beneficial reuseof the sediment is possible.

EXAMPLES

The benchtop experiments described below which demonstrate theapplication of the smoldering process were carried out by placingapproximately 3 kg of soil in a 14 cm (5.5 inch) diameter fused-quartzcolumn. The bottom layer of soil is heated to 300° C. and air is thenintroduced through a sparger at the bottom of the column to ignite asmoldering combustion front. Next, the heater is switched off and thesmoldering front is allowed to propagate upward in the column, fed byair and the fuel available in the soil. Thermocouples at differentheights within the soil bed measure temperatures and soil samples takenbefore and after smoldering are analyzed to determine the effectivenessof contaminant removal. Examples 3 to 6 incorporate findings frombenchtop experiments conducted in the above system to demonstrate theapplicability of the continuous ex situ process to difficult remediationprojects.

Example 1

Biodiesel (“BDSL”) was added to soils at concentrations between 0 and 2weight percent and the prepared soils were subject to benchtopsmoldering tests as described above. The temperature profiles (FIG. 2)show how the addition of biodiesel to the soil causes a transition froman unsustainable process (smoldering front cools as it moves up the soilcolumn) to a self-sustaining one at 1.5 weight percent biodiesel(smoldering front temperatures stabilize above 600° C.). The soilanalysis (FIG. 3) demonstrates that adding sufficient biodiesel to thesoil to achieve a self-sustaining process allows for effective removalof soil hydrocarbons in the range of C16-C34, which includes the addedbiodiesel.

Example 2

Humalite (“Hum”), a material rich in humic acids, was mixed into soilsat concentrations between 0 and 3 dry weight percent. The prepared soilsthen underwent benchtop smoldering tests as described above, whichdemonstrated that the addition of 1.5 dry weight percent humalite wassufficient to establish self-sustaining smoldering (temperaturesstabilize near 600° C. as the front moves up the soil column). Inaddition, increasing the concentration of humalite to 3 dry weightpercent increased the stable front temperature to about 700° C., whichwould enhance contaminant oxidation and reduce harmful emissions. Theresults are shown graphically in FIG. 4.

Example 3

As demonstrated by fireflooding, a smoldering front not only destroyshydrocarbons, but mobilizes them. A benchtop smoldering experiment withan oil-contaminated soil sample showed that some oil migrates ahead ofthe smoldering combustion front (into a layer of ceramic beads in theexperiment). This migration may complicate an in situ application, ascontaminants can accumulate at the periphery of the treatment area. Suchcontaminant “halos” could even require subsequent ex situ treatment ordisposal. In the continuous ex situ system, outward migration of organiccontaminants is limited by the walls of the chamber, which can beinsulated to maximize combustion, and volatile contaminants can becaptured by the air emissions control system. Since non-volatilecontaminants cannot effectively escape the hot zone, a larger proportionmay be destroyed.

Example 4

A clayey soil sample from a site required shredding to achieve adequateair distribution through clayey agglomerates. When tested in thebenchtop unit, smoldering was established in the shredded clayey soilwith a smoldering combustion front advancing with a temperature of 850°C. at a height of 1 cm in the soil column 10 minutes after ignition. Thefront progressed up the column and reached a height of 11 cm 60 minutesafter ignition, with a temperature at that level of 800° C. While suchprocessing is not possible in situ, it can be coupled with an ex situsystem. As shown in FIG. 1, the unit includes a shredder for reducingthe size of feed agglomerates to improve packing and air distribution.Alternatively, such processing could occur in advance without systemintegration.

Example 5

Samples of two contrasting soils from a site in need of remediation weretested in the laboratory as described above. One of the soils was sandyand lacked sufficient fuel for sustaining smoldering combustion, whilethe other was peaty and combusted too vigorously due to its highconcentration of natural organic matter. Neither soil was suitable forin situ remediation by smoldering, but the benchtop experimentsdemonstrated that mixtures of the two soils smoldered in a sustained andcontrolled manner, permitting effective contaminant destruction (FIG.5). In field applications, mixing may be achieved by adding another feedinlet to the continuous ex situ system or in a separate preprocessingstep. The ability to intermix soils before or within the continuous exsitu system may be useful for complex sites with multiple soil types,such as former oil refineries.

Example 6

Some soils have contaminant levels that are too low to sustainsmoldering combustion. If remediation is required, the preferred methodwould be bioremediation, but it may be ineffective if the contaminantsare resistant, e.g., refractory hydrocarbons. In such cases, smolderingcan be applied by adding an environmentally benign source of fuel suchas biodiesel or humalite. In FIGS. 2 and 4, the lowest trace shows thatsmoldering is not maintained beyond a short distance from the point ofignition without the benefit of a smoldering-promoting additive. Theaddition of 2% biodiesel or 2% humalite, however, enables a stablesmoldering front to pass through the soils with temperatures above 600°C. As in earlier examples, this approach is not compatible with prior insitu methods. However, using the continuous ex situ system, a solid fuelcan be mixed in similarly to a second soil (Example 5), while a liquidfuel can be dispersed with a spray nozzle at the top of the smolderingchamber.

Without a strategy for increasing the fuel content of such soils,remediation by smoldering would not be viable since the smoldering frontfails to sustain itself. In addition, strategies that rely oncontaminant-type fuel sources (e.g., oily waste or petroleumhydrocarbons) effectively exacerbate the level of pollution beforeattempting to remediate, which carries with it a risk of incompleteremoval of the contaminant fuel source and potentially hazardousemissions generation. Therefore, smoldering-promoting additives mayprove beneficial for particulate materials with low levels ofcontaminants that are resistant to bioremediation.

Example 7

A site formerly subject to extensive petroleum industry operations hashundreds of kilotonnes of soil requiring remediation. The site is alsoon the edge of a forest with soil rich in peat, a smoldering promoter,which poses a substantial risk if an in situ smoldering remediation wereto spread beyond the area of intended confinement. This risk ismitigated by the use of the continuous ex situ smoldering unit, assmoldering soil is isolated from the surroundings.

Example 8

Oily sludge from a refinery was found to have excessive combustiblecontent (approximately 40 weight percent total organic carbon) for acontrolled smoldering process. To apply smoldering to treat the sludge,dilution in a less combustible particulate material (e.g., sand orsmoldered soil) would be required. Treatment of this sludge bysmoldering would not be possible in an in situ format. However, thesludge is amenable to treatment in the continuous ex situ system, inwhich blending with a less combustible particulate material could occurprior to or during admission to the smoldering chamber. In addition,while blending with a less combustible particulate material decreasestreatment throughput for the sludge, the continuous ex situ systembetter compensates for this drop in throughput by eliminating theloading-heating-smoldering-cooling-emptying cycles associated with batchoperation.

The invention claimed is:
 1. A process for the application of smolderingcombustion to remediate contaminated particulate materials, comprising:introducing up to 10 weight percent of a solid additive into acontaminated particulate material to enable or enhance smolderingcombustion, wherein the solid additive has a total organic carbonconcentration of at least 25 weight percent, and wherein the solidadditive comprises a soil conditioner comprising peat, leonardite,humalite, biochar, biosolids, compost, or combinations thereof; andinitiating smoldering combustion in the contaminated particulatematerial.
 2. A process according to claim 1, wherein the additiveenables a contaminated particulate material that is unable or onlyintermittently able to sustain smoldering combustion to consistentlysustain smoldering combustion.
 3. A process according to claim 1,wherein the additive is mixed into the contaminated particulate materialprior to or during transfer of the contaminated particulate material forex situ smoldering treatment.
 4. A process according to claim 3, whereinthe ex situ smoldering treatment occurs within the confines of asmoldering unit.
 5. A process according to claim 3, wherein the ex situsmoldering treatment occurs in an engineered mound or pit.